The calcium oxide and magnesium oxide are very reactive to water, and hence are utilized as moisture adsorbing materials. Further, both of the calcium oxide and magnesium oxide are basic oxides and very reactive to acids, and hence are utilized as materials for (chemically) adsorbing acidic gases such as a hydrogen fluoride gas, a hydrogen chloride gas, a sulfur dioxide gas and a carbon dioxide gas. Furthermore, studies have been made recently on the calcium oxide and magnesium oxide for utilizing them as materials for adsorbing decomposition products of halogenated hydrocarbon gases, such as, a fluorocarbon gas which is used in a process for manufacture of semiconductor materials, and a halogen gas which is employed as fire extinguisher material.
Japanese Patent Provisional Publication 7-149580 A describes highly active porous calcium oxide granules having high reactivity to a carbon dioxide gas, which are porous calcium oxide granules obtained by calcining granules of calcium hydroxide or calcium carbonate having a specific surface area of at least 5 m2/g and a particle size of at least 1 mm. This publication describes that the highly active porous calcium oxide granules are produced by calcining granules of calcium hydroxide powder having a size of 300 μm or less at an increasing temperature from 390 to 480° C. for at least 5 minutes, or by calcining granules of calcium carbonate powder having a size of 300 μm or less at an increasing temperature from 700 to 780° C. for at least 5 minutes.
Japanese Patent Provisional Publication 2002-224565 A proposes a material for decomposing a fluorocarbon gas which comprises a granular catalyst (aluminum oxide) for decomposing a fluorocarbon gas and granules of alkaline earth metal oxide such as calcium oxide or magnesium oxide. This publication describes that the fluorocarbon gas-decomposing material is regenerated to give aluminum oxide by reacting aluminum fluoride produced by the reaction of the fluorocarbon gas with aluminum oxide with an alkaline earth metal oxide, and that hence the fluorocarbon gas can be decomposed continuously for a long period of time. In the working examples of this publication, the granules of alkaline earth metal oxide are produced by molding powdery alkaline earth metal oxide under pressure.
“Effects of Composition of Solid Adsorbing Material on Adsorption of Halon Decomposition Gas” by TAKEUCHI, Akihiro, et al., Journal of the Society of Inorganic Materials, Japan, 12, 97-105 (2005) reports that when a halogenated hydrocarbon gas (Halon 1301 gas) is brought into contact with calcium oxide, magnesium oxide, or a mixture of calcium oxide and magnesium oxide at approx. 900° C. (1173K), a decomposition product (fluorine, bromine) of the halogenated hydrocarbon gas is more highly adsorbed by the mixture of calcium oxide and magnesium oxide than by calcium oxide or magnesium oxide alone. In this publication, the mixture of calcium oxide and magnesium oxide is produced by kneading a mixture of calcium hydroxide and magnesium hydroxide in a pure water, molding the kneaded product, and calcining the molded product at approx. 1,000° C. (1273K) in an electric furnace.
It is preferred that a granular material comprising calcium oxide or a granular material comprising calcium oxide and magnesium oxide to be employed as a moisture adsorbing material or a material for adsorbing gases such as an acidic gas or a decomposition product of a halogenated hydrocarbon gas has a large contact surface area (i.e., BET specific surface area) with a gas to be absorbed. Further, it is preferred that the granular gas-adsorbing material has high resistance to physical shock and shows excellent form retention. The reasons are as follows. The granular gas-adsorbing material is generally charged in a gas-processing apparatus such as a column. If the granular gas-adsorbing material powders (is broken down) in the course of the adsorbing material-charging procedure or in the course of the gas-adsorbing operation (in the time when a gas is brought into contact with the gas adsorbing material), the pressure loss in the gas-processing apparatus increases.